Functional fluids

ABSTRACT

This invention is for improvements in or relating to hydraulic fluids and is particularly concerned with liquids employed as the power-transmitting medium in hydraulic brake systems for vehicles, for example, motor cars and airplanes. The hydraulic fluid of the invention contains at least 50 percent by weight of at least one ester of the formula   WHEREIN R is an alkylene group of from two to eight carbon atoms, R1 is an alkyl of from one to six carbon atoms or a phenyl radical, R2 is ethylene, propylene or butylene and n is an integer of from zero to three.

United States Patent lnventors Robert Alan Cameron Ker;

Robert Carswell; John Lavington Clarke; Peter Anthony Burt, all of London, England 3,553

Jan. 8, 1970 Nov. 30, 1971 Costrol Limited London, England June 18, 1964 Great Britain Appl. No. Filed Patented Assignee Priority FUNCTIONAL FLUIDS 19 Claims, No Drawings US. Cl 252/79, 252/56, 252/77, 252/79 Int. Cl C09k 3/02 Field of Search 252/56, 73, 75, 76, 79; 260/485 G References Cited UN lTED STATES PATENTS 2,916,457 10/1959 Friedrich et al. 252/75 3,218,256 11/1965 Edwards et a1. 252/475 3,274,113 9/1966 Reiland 252/78 2,629,693 2/1953 Barton et al. 252/334 2,542,785 2/1951 Walker 252/79 Primary Examiner- Leon D. Rosdol Assistant Examiner-P. E. Willis /C O OR (O RDnOR R COORKORDnOR wherein R is an alkylene group of from two to eight carbon atoms, R' is an alkyl of from one to six carbon atoms or a phenyl radical, R is ethylene, propylene or butylene and n is an integer of from zero to three.

FUNCTIONAL FLUIDS This is a continuation of a application Ser. No. 464,21 1 filed June 15, 1965, now abandoned BACKGROUND OF THE INVENTION Hydraulic brake systems comprise metal and rubber parts which are exposed to the liquid employed as the power transmitting medium and it is essential that a liquid be used which has little, if any attack on either the metal or the rubber parts of the brakesystem, either at ordinary temperatures or at the elevated temperatures which are setup in the parts of the system adjacent to the braking surfaces during a prolonged application of the brakes. It is further more essential that the hydraulic transmission medium shoulder be stable and of a high boiling point such as not to vaporize to any substantial extent under operating conditions. Again, the liquid should have a low freezing point to withstand operating temperatures which obtain either during cold weather, or in the case of aircraft, during high altitude flight. in addition it is necessary that the liquid shall have a sufficiently low viscosity to render the system readily operable, the viscosity not changing to any material extend under operating conditions. Finally further important considerations are that the liquid shall have a low rate of vaporization and also sufficient lubricating properties properly to lubricate the moving parts of the system.

In the hydraulic systems of motor care typical heavy duty" brake fluids in service at the present time boil at about 216 C. With the generally increasing severity. of operation, however, especially in vehicles fitted with disc brakes, there is a need for hydraulic fluids having even higher boiling points and fluids boiling at about 260 C. are now being called for. Fluids of this latter type in commercial use normally comprise a major proportion of one or more polyethylene glycols, ethers or polyethylene glycols, or mixtures of such compounds. Minor proportions of castor oil or lubricity additives, such as polyalkylene glycol polyesters, may be present, together with one or more corrosion inhibitors.

In practice, it is found that hydraulic brake fluids based on polyethylene glycols and/or ethers thereof, are hygroscopic an in process of time water is absorbed from the atmosphere and the boiling point steadily falls. Thus, after a period of use, two brake fluids having initial boiling points more than 56 C. apart may be found to have boiling points of the same order, both substantially below the initial boiling points of the fluids. It is not uncommon, for example, for a brake fluid, after several months service, to be found to contain about 1 percent of water and, on prolonged service, up to 3 percent of water may be found in such fluids.

In a simple laboratory experiment, a typical known hydraulic brake fluid, fluid A, having an initial boiling point of 216 C., was found to have a boiling point of 182 C. when l percent of water was added and a boiling point of only 141 C. when 3 percent of water was present. A known high-temperature brake fluid, fluid B, which had an initial boiling point of 271 C. was found to have a boiling point of 204 C. when 1 percent of water was present and of only 154 C. when 3 percent of water was present. The high temperature brake fluid, therefore, had become inferior to the conventional brake fluid in its original condition as a result of absorbing only 1 percent of water. 7

In another simple laboratory test in which 150 mls. of fluid were placed in open 250 ml. beakers and exposed to the atmosphere of the laboratory, it was found that the boiling point of the conventional brake fluid, Fluid A, fell from 216 C. to 137 C. in 4 days, whereas the boiling point of the high-temperature brake fluid, fluid B, fell from 271 C. to 146 C. in 4 days. The advantage of fluid B over fluid A was thus dissipated in a very short time.

it is important for a hydraulic brake fluid to retain a high boiling point throughout its useful life since a reduction in boiling point may result in the formation of vapor locks with consequent failure of the braking system. In order to avoid this possibility, it is necessary for current high-temperature brake fluids based on polyglycols and/or plyglycol ethers, to be changed fairly frequently in the braking system, so that moisture is not allowed to accumulate. It is desirable that the boiling point of a fluid should not be allowed to fall below about 149 C.

We have found that high-temperature hydraulic fluids, which have relatively low hygroscopicity and high boiling points, may be formulated from certain esters of organic .dicarboxylic acids and ethers of certain glycols.

The invention provides a hydraulic fluid having a kinematic viscosity at 40 C. of not more than 5,000 centistokes, preferably not more than 2,000 centistokes, and preferably having a boiling point of at least 260 C., and either consisting of or comprising at least 50 percent by weight, preferably at least percent of at least one ester having the general formula (JOORMORDnOR where R is a straight or branched chain alkylene group which contains at least two, preferably from two to eight carbon atoms, R is an alkyl radical having from one to four preferably one to two carbon atoms or a phenyl radical R is anethylene, propylene or butylene group, and n is O, l 2 or 3, together with a minor amount of at least one additive. The groups R- R and n may be the same or different.

Preferred acids R(COOH from which the esters are derived are succinic, glutaric,- adipic, azelaic, sebacic and isosebacic acids, although other dicarboxylic acids may be used, if desired, c.g. a commercially available mixture of adipic, glutaric and succinic acids, known as A.G.S. acid." Preferred examples of the glycol ethers from which the esters are made are:

Ethylene glycol monomethyl ether (Methyl Oxitol" or Methyl Cellosolve").

Diethylene glycol monoethyl ether (Dioxitol or "Carbitol).

Diethylene glycol monomethyl ether (Methyl Dioxitol" or Methyl Carbitol).

Triethylene glycol monomethyl ether (Methyl Trioxitol).

Ethylene glycol monoethlyl ether (Ethyl Oxitol).

Triethylene glycol monoethyl ether (Ethyl Trioxitol).

Preferred esters in accordance with the present invention are given in table 1 together with their physical properties, it being understood that when the esters have too high a viscosity, particularly at -40 C., then they may be blended with a suitable ester or a small amount of inert diluent to give a fluid having the viscosity requirements in accordance with the present invention. For example a blend of 15 percent di(ethyl trioxitol) sebacate and percent (methyl oxitol adipate had the following viscosity characteristics:

1,006 cS. 40 C.

8.76 cS. cS. c80140 38 C.

2.63 cS. 99 C.

Typical additives which may be used in the invention are lubricity additives selected from castor oil or castor oil treated in various ways, for example,

Firsts Castor oil,

Castor oil to specification DTD72 Blown castor oil, i.e. castor oil blown with air or oxygen while being heated.

Special Pale Blown Castor i.e. a similarly blown castor.

Hydricin 4" i.e. a commercially available ethylene/oxide/propylene oxide treated castor oil.

Other lubricity additives which may be incorporated in hydraulic fluids in accordance with the present invention include borate esters e.g. tricresyl borate and phosphorus-containing esters, especially phosphates e.g. tricresyl phosphate.

The hydraulic fluids of the present invention may also include minor proportions of polyexyalkylene or ethers thereof e.g. those sold under the Reg. Trade Mark Ucon, particularly those of the LB and HB series. Suitable examples of these polyoxyalkylene glycols and their ethers and esters are given in our copending application No. 337,181 filed Jan. 13, 1964 now U.S. Pat. No 3,334,048 together with examples of other suitable lubricity agents, such as orthophosphate or sulfate salts of primary or secondary aromatic amines, dialkyl citrates, aliphatic dicarboxylic acids and esters thereof, specific examples being Diamylamine orthophosphate Dinonylamine orthophosphate Diamylamine sulphate Dinonyl citrate Di (2-ethyl hexyl) citrate Polyoxyethylene sebacate derived from a polyaxyethylene glycol of M.W. 200

Polyoxethylene azelate derived from polyoxyethylene glycol of M.W. 200

Polyoxyethylene polyoxypropylene glutarate derived from mixed polyoxyglycols of average M.W. of about 200 Glutaric acid Axelaic acid Sebacic acid Succinic acid Di ethyl sebacate Di 2-etl1yl hexyl sebacate Di iso octyl azelate Unsaturated aliphatic acids or their salts may also be used e.g. oleic acid or potassium ricinoleate.

Corrosion inhibitors which may be used in the present invention may be selected from heterocyclic nitrogen-containing compounds e.g. benzotriazole and benzotriazole derivatives such as those described in British Pat. specification No. 1,061904 or mercapto benzothiazole. Many amines or derivatives thereof are also suitable as corrosion inhibitors, for example di n-butylamine di n-amylamine cyclohexylamine morpholine triethanolamine and soluble salts thereof e.g. cyclohexylamine carbonate.

Phosphites are also good corrosion inhibitors e.g.

Tri phenyl phosphite Di isopropyl phosphite and certain inorganic salts may be incorporated e.g. sodium nitrate.

Other additives which may be included are antioxidants such as diarylamines e.g. diphenylamine, p.p. 'dioctyldiphenylamine, phenyl-a-naphthylamine or phenyl-B- naphthylamine. Other suitable antioxidants are those commonly known as hindered phenols which are exemplified by 2,4dimethyl-6-t-butyl phenol 2,6 di-t-butyl -4-methyl phenol 2,6 di-t-butyl phenol 1,1 bis( 3 5 di-t-butyl-4-hydroxyphenyl)methane 3,3 ',5,5 tetra-t-butyl-4-4'-dihydroxydiphenyl 3-methyl 4,6di-t-butyl phenol 4-methyl-2-t-butyl phenol Yet further additives which may be used are phenothiazine and its derivatives, for example those having alkyl or aryl, groups attached to the nitrogen atom or to the aryl groups of the molecule.

These additives will normally be employed in conventional amounts which, in many cases, will be from 0.05 percent to percent, for example fonn 0.1 percent to 2 percent by weight.

In formulating brake fluid compositions to meet the requirements of SAE 70R3 Specification it is necessary carefully to select R, R and R in the ester in conventional manner so that the fluid has the desired physical properties and meets the rubber swell requirement of the specification. While in some cases it may be possible to use one of the foregoing esters by itself, or a mixture of two or more such esters it will usually be necessary to modify the action of the ester or esters on rubber by incorporating a minor proportion of one or more polyoxyglycol ethers. The addition of such polyoxyglycol ethers also improves the water tolerance of the fluid which is one of the requirements of SAE 7OR3 specification. Thus, for example, di(methy1 oxitol) succinate by itself failed the water tolerance test at --40 C., but this difficulty was overcome by incorporating 10 percent of mixed polyoxyethylene glycol ethers. This water tolerance test is carried out by adding 3.5 percent of water to the composition and holding it at -40 C. for 24 hours; no ice formation or cloudiness must occur.

The invention also provides a hydraulic system containing a hydraulic fluid consisting of, or comprising a major proportion, preferably at least percent of at least one ester having the general formula:

/COOR (OR )nOR ooorwononoa where R, R, R and n are as defined above. Additives as described above may be included in minor proportions in the fluid. Alternatively the fluid may consist entirely of one or more 0 the esters with no additons. The invention also includes a method of operating a hydraulic system which method comprises employing a hydraulic fluid consisting of, or comprising a major proportion, preferably at least 80 percent, of the ester defined above.

The effect on rubber of typical esters of the present invention is shown in tables 11 and 111. The results given in table 11 were those obtained when a piece of a natural rubber of the type normally used in hydraulic and brake systems was immersed in the fluid under test at 70 C. The percentage volume swell was determined by using the Archimedean principle, the rubber being weighed while suspended in water before and after test. Results of similar tests carried out using a synthetic rubber of the type normally used in hydraulic systems are given in table Ill. In the case of natural rubber no particular limit is set but the nonnally accepted maximum is 5 percent and in the case of synthetic rubber the upper limit set by some manufacturers of brake systems is 15 percent. In both cases the rubber should not be unduly dry and should not be cracked or unduly soft. All the esters shown were satisfactory in this respect.

The following examples illustrate typical hydraulic fluid compositions in accordance with the present invention:

EXAMPLE 1 99 percent Di(methyl oxitol) succinate 1 percent Primene 81-R (Reg. Trade Mark for a tertiary alkyl primary amine having 12 to 14 carbon atoms).

This composition had viscosities of 1,680 cS. at -40 C., 3.75 cS. at 50 C. and 1.59 cS. at 100 C., and a boiling point of 292 C.

This composition, although representing a useful brake fluid, failed the SAE 70R3 Specification in respect of the 40 C. water tolerance clarity clause.

EXAMPLE 2 87.9 percent Di( methyl oxitol) succinate 10 percent Mixed polyoxyethylene glycol ethers 1 percent Ucon 50 HB 5100 0.1 percent Benzotriazole 1 percent Primene 81R This composition had viscosities of 1,640 08. at 40 C. 4.2 cS. at 50 C. and 1.68 cS. at 100 C. and in initial boiling point of 282 C. It also passed the SAE 7OR3 water tolerance and low temperature requirements.

In this blend the object of the Ucon SOHB 5100, a polyoxyethylene polyoxypropylene glycol ether, was to increase the viscosity, while the benzotriazole was to inhibit corrosion of copper. The amine was present to raise the pH in accordance with the SAE 70R3 limits.

In two laboratory hygroscopicity tests carried out at different times, as previously described, 150ml. samples of hydraulic fluids were placed in open 250 ml. beakers and exposed to the atmosphere of the laboratory for 3 to 6 days. Boiling points were measured before and after exposure. Fluids A and B are the typical prior art fluids discussed in the preamble. The following results were obtained The excellent low-hygroscopicities of vention are further demonstrated with reference to examples 3-8 in table IV, a direct comparison having been made with fluid B of the prior art. I

Two further examples of corrosion-inhibited compositions are given below (example 9 and 10 EXAMPLE 9 percent Di( methyl trioxitol) adipate 60 percent Di( methyl oxitol) nylonate 14 percent Ucon HB 1 percent Reoplex 400 0.5 percent Di-n-butylamine 0.1 percent Benzotriazole EXAMPLE 10 40 percent Di(methyl trioxitol) adipate 53.9 percent Di( methyl oxitol) adipate 5 percent Ucon H.436

0.5 percent Di-n-butylamine 0.5 percent Phenyl-a-naphthylamine 0.1 percent Benzotriazole The blends of examples 9 and 10 were subjected to the full test laid out in the S.A.E. R3 specification as drawn up by the Society of Automotive Engineers.

The fluids were entirely satisfactory apart from the boiling point change and the pH after corrosion. However these results were not considered to be detrimental in view of the fact that the boiling point after test was far in excess of that of any current fluid and the pH after corrosion was irrelevant provided that the corrosion test was satisfactory in other respects.

EXAMPLE 1 1 39.9 percent Di(methyl trioxitol) adipate 5 3.9 percent Di( methyl oxitol) adipate 5 percent Ucon H 436 0.5 percent Di-n-butylamine 0.5 Phenyl-B-naphthylamine 0.1 percent Benzotriazole 0.1 percent Phenothiazine This fluid, which is very similar to that of example 10, passed all the tests of the S.A.E. 70R3 Specification. The phenothiazine appeared to act in controlling the pH, and in further reducing corrosion.

The hydraulic fluids of this invention as described in the examples are suitable for use under high temperature conditions, have relatively low hygroscopicity and retain high boiling 35 points over extended periods of use.

] TAQLE I Eater Viscosities 05 at SA!) 70113 -4oc. 38C. 99c. 1 311 3 3 Di (methyloxitol) Succinate 1565 5.26 1.55 293 D1 (methyl dioxitol) Succinate 8310 10.8 2.56 298 D1 (methyl trioxitol) Succinate Too thick 24.25 4.63 303 Di (ethyl oxitol) Succinate 1870 5.33 1.59 287 Di (ethyl dioxitol) Saccinate 14,510 11.22 2.69 277 Di (ethyl trioxitol) Succinate 'loo thick 18.65 4.06 321 Di (methyl oxitol) Adipate 534 5.63 1.'/6 293 Di (methyl dioxitol) Adipats 2005 9-78 2.73 308 Di (methyl trioxitol) .Adipate 11,000 13.89 3.43 Di (ethyl oxitol) Adipate 696.3 5.95 1.83 316 D1 (ethyl dioxitol) Adipate 2458 9.94 2.70 31 Di (cthyl trioxitol) Adipate 11,704 17.95 2.95 31 D1 (methyl oxitol) Sebacate solid 8.26 2.38 32 Di (methyl dioxitol) Sebacate solid 14-43 3.69 302 Di (methyl trioxitol) Sebacate solid 26.00 5.89 307 Di (ethyl oxitol) Sebacate solid 8.73 2.55 340 Di (ethyl dioxitol) Sebacate solid 10.35 4.03 314 Di (ethyl trioxitol) Sebacate solid 27.00 5.94 316 Di (methyl oxitol) Azelate 812 8.27 2-38 33 Di (methyl dioxitol) Azelate 2490 12-99 3.39 31 Di (methyl trioxitol) llzelate 9,660 24.51 5.58 3 Di (ethyl oxitol) Azelate solid 8.44 2.75 335 Di (ethyl dioxitol) Azelate 2135 11.78 3.18 284 Di (ethyl trioxitol) Azelate 6824 19.73 4.75 298 +Di (methyl oxitol) Nylonate 647 5.23 1. 7 2

+112: ester prepared by reacting ethylene glycol mono methyl ether with a mixture known as "A.G.S. Acid" containing approximately-15596 adipic acid,

and

% gluturic acid and 5,35% succinic acid. The material is supplied by Imperial Chemical Industries Limited.

TABLE l 7 I where R is an alkylene group of two to eight carbon atoms,

R is selected from the group consisting of an alkyl radical of one to four carbon atoms and a phenyl radical,

% volume Increase on Natural Rubber at C. R is selected from the group consisting of ethylene,

propylene and butylene, and 2 I n is an integer offrom 0 to 3; and mid b. an organic antioxidant;

said component (a) being present in the mixture in an t y oxitol) succimte 236 1-26v amount of at least 50 percent by weight based on the total 3S521233223223232.3221 1:2: :5: weight Me and components 5 and a e mmflhyl omen Adipm so selected to provide a mlxture havlng a kinemat c Di(methyl trioxitol) Adipate 0.3l 1.53 viscosity at -40 C. of not more than 5,000 c5. and a boll- Di(ethyl trioxitoUAdipate l.l7 2.82 ing point f at least 2 0 2. In a method of hydraulic transmission of power to pressure operated elements the improvement which comprises employing as the hydraulic fluid a composition consisting essentially of a mixture of the following components:

TABLE m a an ester or mixture of esters of the formula COORKOR MOR Ester '1 Volume increase synthetic rubber 3 days at I20 C. R\

COORHORUnOR Di(methyl dioxitol) Succinate 7.48 Ditmethyl trioxitol) Succinate 4.72 Dflmgth l "tome" Adipm 244 where R is an alkylene group of two to elght carbon atoms. iw y l n t 852 R is selected from the group consisting of an alkyl radical i'gi' gs 13$; :2; of one to four carbon atoms and a phenyl radical, Dkmcnwl maxim) sebum M R is selected from the group consisting of ethylene, Di (methyl trioxitol) Azelate 2.65 propylene and butylene, and Dl(ethyl trloxltol) Azelatc 7.62 n i an i g offrom 0 to 3; and +Dl( methyl oxltol) Nylonate 13.20

b. an organic corrosion inhibitor;

said component (a) being present in the mixture in an amount of at least 50 percent by weight based on the total weight of the mixture and components (a) and (b) being +An ester prepared by reacting ethylene glycol mono methyl ether with a mixture known as A.G.S. Acid and containing approximately 15-25 percent adipic acid,

-60 percent glutaric acid and 25-35 percent succinic acid. Selected to P id a mixture having a kinematic TABLE IV Hygroscopicity Tests Boiling point C. at Boiling point C. after Ex. No. Blend Initial 1 day 2 days 3 days 4 days 5 days 6 days 3 80% di(methyloxitol) nylonate; 207 special castor 294 284 239 221 186 4. 80% di (methyl oxitol) succinate; 20 0 special castor. 292 270 242 5. 80% di(methyl oxitol) adlpate; 20% special castor. 297 287 234 Fluid B 268 221 161 Percent humidity in the above four tests 48. 5 46 61. 5

6 8% MB castor; 12% special caster; 80% di(methyl oxitol) nylonate 279 238 7. 5% Ucon 5011B 5100; 2% Reoplex 400; 93% di(methyl oxitol) nylonate. 280 254 8 4% Reoplex 400: 96% di(methyl oxitol) nylonate 282 240 Fluid B. 268 209 N era:

The Special Castor was an ethylene oxide/propylene oxide treated castor. MB castor is a heat treated air blown castor oil. Rcoplex 400 was a polyoxyethylene glycol adipate of molecularweight 400 marketed by Geigy Ltd. Ucon 5011B 5,100 was a polyoxyethylene polyoxypropylenc glycol ethermarketed by Union Carbide and having a viscosity of about 5,100 S. U.S. at

We claim: viscosity at 40 C. of not more than 5000 0S. and a boil- In a method of hydraulic transmission of power to presg P ofa! 163$! sure operated elements the improvement which comprises In a method of hy r n m n f power to pr employing as the hydraulic fluid a composition consisting ess r per lem n the Impr vem nt wh h ompri sentially of a mixture of the following components: employing as the hydraulic fluld a composition consisting esa. an ester or mixture of esters of the formula sentially of a mixture of the following components;

a. an ester or mixture of esters of the formula COORHORDHOR COOR (OR )HOR R R\ COOR (OR )nOR COORKOR MOR where R is an alkylene group of two to eight carbon atoms,

R is selected from the group consisting of an alkyl radical of one to four carbon atoms and a phenyl radical,

R is selected from the group consisting of ethylene,

propylene and butylene, and

n is an integer of from to 3; and

b. a lubricity agent selected from the group consisting of castor oils, triaryl borates, triaryl phosphates, polyoxyalkylene glycols and ethers thereof, orthophosphates of primary and secondary amines, sulfates of primary and secondary amines, dialkyl citrates, saturated aliphatic dibasic carboxylic acids and alkyl esters thereof, and unsaturated aliphatic acids and their salts;

said component (a) being present in the mixture in an amount of at least 50 percent weight based on the total weight ofthe mixture and components (a) and (b) being so selected to provide a mixture having a kinematic viscosity at 40 C. of not more than 5000cS. and a boiling point of at least 260 C.

4. In a method of hydraulic transmission of power to pressure operated elements, the improvement comprising employing as the hydraulic fluid a mixture consisting essentially of esters of the formula where R is analkylene group of two to eight carbon atoms,

R is selected from the group consisting of an alkyl radical of one to four carbon atoms and a phenyl radical,

R is selected from the group consisting of ethylene,

propylene and butylene, and

n is an integer of from 0 to 3, said ester mixture containing a major proportion of esters prepared by reacting ethylene glycol monomethyl ether with a mixture of dibasic acids containing approximately -25 percent adipic acid, 45-60 percent glutaric acid and 25-35 percent succinic acid, and a minor proportion of di(triethylene glycol monomethyl ether) adipate.

5. The method of claim 3 wherein said composition includes an organic antioxidant.

6. The method of claim 3 wherein said composition includes an organic corrosion inhibitor.

7. The method of claim 3 wherein said composition includes an organic antioxidant and an organic corrosion inhibitor.

8. The method of claim 3 wherein component (b) of said composition is a polyoxyalkylene glycol or ether thereof in an amount of from about 5 percent to about 14 percent by weight of said composition.

9 The method of claim 3 wherein R of the formula for said ester component (a) is selected from the group consisting of methyl and ethyl.

10. The method of claim 3 wherein R is methyl.

1 l. The method of claim 3 wherein R is ethylene.

12. The method of claim 3 wherein R is an alkylene group of two to four carbon atoms.

13 The method of claim 3 wherein said ester or mixture of esters is derived for an acid selected from the group consisting of succinic, glutaric, adipic, azelaic, sebacic, and isosebacic acids and mixtures thereof.

14. The method of claim 3 wherein component (a) is a mixture of esters of the recited formula, said mixture of esters being derived from a mixture of esters consisting of adipic acid, glutaric acid and succinic acid.

15. The method of claim 14 wherein the mixture of acids consists of approximately 15 to 25 adipic acid, 45 to 60 percent glutan'c acid and 25 to 35 percent succinic acid.

16. The method of claim 3 wherein the component (a) derived from a glycol ether selected from the group consisting of methyl and ethyl ethers of mono-, di, and triethylene glycols.

17. The method of claim 3 wherein said component (a) comprises a mixture of esters prepared by reacting ethylene glycol monomethyl ether with amixture of dibasic acids consisting of 15 to 25 percent adipic acid, 45 to 60 percent glutaric acid, and 25 to 35 percent succinic acid.

18. The method of claim 17 wherein said component (a) additionally comprises di(triethylene glycol monomethyl ether) adipate.

19. The method of claim 3 wherein said component (a) is present in the mixture in an amount of at least percent by weight based on the total weight of the mixture. 

2. In a method of hydraulic transmission of power to pressure operated elements the improvement which comprises employing as the hydraulic fluid a composition consisting essentially of a mixture of the following components: a. an ester or mixture of esters of the formula where R is an alkylene group of two to eight carbon atoms, R1 is selected from the group consisting of an alkyl radical of one to four carbon atoms and a phenyl radical, R2 is selected from the group consisting of ethylene, propylene and butylene, and n is an integer of from 0 to 3; and b. an organic corrosion inhibitor; said component (a) being present in the mixture in an amount of at least 50 percent by weight based on the totAl weight of the mixture and components (a) and (b) being so selected to provide a mixture having a kinematic viscosity at -40* C. of not more than 5000 cS and a boiling point of at least 260* C.
 3. In a method of hydraulic transmission of power to pressure operated elements the improvement which comprises employing as the hydraulic fluid a composition consisting essentially of a mixture of the following components: a. an ester or mixture of esters of the formula where R is an alkylene group of two to eight carbon atoms, R1 is selected from the group consisting of an alkyl radical of one to four carbon atoms and a phenyl radical, R2 is selected from the group consisting of ethylene, propylene and butylene, and n is an integer of from 0 to 3; and b. a lubricity agent selected from the group consisting of castor oils, triaryl borates, triaryl phosphates, polyoxyalkylene glycols and ethers thereof, orthophosphates of primary and secondary amines, sulfates of primary and secondary amines, dialkyl citrates, saturated aliphatic dibasic carboxylic acids and alkyl esters thereof, and unsaturated aliphatic acids and their salts; said component (a) being present in the mixture in an amount of at least 50 percent weight based on the total weight of the mixture and components (a) and (b) being so selected to provide a mixture having a kinematic viscosity at -40* C. of not more than 5000cS and a boiling point of at least 260* C.
 4. In a method of hydraulic transmission of power to pressure operated elements, the improvement comprising employing as the hydraulic fluid a mixture consisting essentially of esters of the formula where R is an alkylene group of two to eight carbon atoms, R1 is selected from the group consisting of an alkyl radical of one to four carbon atoms and a phenyl radical, R2 is selected from the group consisting of ethylene, propylene and butylene, and n is an integer of from 0 to 3, said ester mixture containing a major proportion of esters prepared by reacting ethylene glycol monomethyl ether with a mixture of dibasic acids containing approximately 15-25 percent adipic acid, 45-60 percent glutaric acid and 25-35 percent succinic acid, and a minor proportion of di(triethylene glycol monomethyl ether) adipate.
 5. The method of claim 3 wherein said composition includes an organic antioxidant.
 6. The method of claim 3 wherein said composition includes an organic corrosion inhibitor.
 7. The method of claim 3 wherein said composition includes an organic antioxidant and an organic corrosion inhibitor.
 8. The method of claim 3 wherein component (b) of said composition is a polyoxyalkylene glycol or ether thereof in an amount of from about 5 percent to about 14 percent by weight of said composition. 9 The method of claim 3 wherein R1 of the formula for said ester component (a) is selected from the group consisting of methyl and ethyl.
 10. The method of claim 3 wherein R1 is methyl.
 11. The method of claim 3 wherein R2 is ethylene.
 12. The method of claim 3 wherein R is an alkylene group of two to four carbon atoms. 13 The method of claim 3 wherein said ester or mixture of esters is derived from an acid selected from the group consisting of succinic, glutaric, adipic, azelaic, sebacic, and isosebacic acids and mixtures thereof.
 14. The method of claim 3 wherein component (a) is a mixture of esters of the recited formula, said mixture of esters being derived from a mixture of esters consisting of adipic acid, glutaric acid and succinic acid.
 15. The method of claim 14 wherein the mixture of acids consists of approximately 15 to 25 percent adipic acid, 45 to 60 percent glutaric acid and 25 to 35 percent succinic acid.
 16. The method of claim 3 wherein the component (a) derived from a glycol ether selected from the group consisting of methyl and ethyl ethers of mono-, di-, and triethylene glycols.
 17. The method of claim 3 wherein said component (a) comprises a mixture of esters prepared by reacting ethylene glycol monomethyl ether with a mixture of dibasic acids consisting of 15 to 25 percent adipic acid, 45 to 60 percent glutaric acid and 25 to 35 percent succinic acid.
 18. The method of claim 17 wherein said component (a) additionally comprises di(triethylene glycol monomethyl ether) adipate.
 19. The method of claim 3 wherein said component (a) is present in the mixture in an amount of at least 80 percent by weight based on the total weight of the mixture. 